A stator to be used in a motor has a core end part insulated and protected by a varnish impregnating method in which varnish is impregnated between wire cores each being wound on a stator core and then the varnish is heated to be solidified or by a resin-molding method in which the core end part of the stator is injection-molded with resin by use of a die to fill insulating resin between wire cores.
As the method of impregnating varnish between the wire cores, there are known an impregnating method conducted by dropping varnish while rotating the stator and a dipping method conducted by dipping a lower part of each wire coil in a varnish bath to impregnate varnish by capillary phenomenon.
Both cases need, before and after a molding process, a pre-heating and drying process for removing moisture from each wire coil exposed in each coil end part of the stator and reducing winding stress (annealing process) and a stator heating process for heating each wire coil to solidify the varnish.
When the coil end parts of the stator are to be resin molded, similarly, pre-heating is required.
Resin to be used in the resin-molding is generally a heat-hardening type. This is because, when the stator is inserted in a die and injection-molded, it will take much time to harden the resin unless the stator is heated in advance. Even when a thermoplastic resin is used, flowability thereof tends to remarkably deteriorate during resin molding unless the stator is heated in advance. This may cause a defect such as insufficient fill.
From the above reasons, even when the coil end parts are resin molded, the stator has to be pre-heated in advance uniformly in a constant temperature range. The resin to be used in resin-molding may include unsaturated polyester resin, PPS resin, LCP resin, etc.
The stator is constituted of a stator core and wire coils mounted in the stator core. Recently, in some cases, a thin-sheet laminated coil, an edgewise wound coil, and other coils are used instead of the wire coil.
Ideally, such stator is uniformly heated in the aforementioned pre-heating process. This pre-heating of the stator is disclosed in many documents.
For example, JP60(1985)-82050A discloses a method of induction-heating using a heating coil and pre-heating a stator by irradiation of infrared rays.
However, in the method in which the stator is inserted and heated in a hollow heating coil as disclosed in JP60-82050A, most of magnetic flux generated from the heating coil would act on outer peripheral end parts of the core. This results from that the stator core is often formed of a laminated body of silicon steel sheets, which is lower in heat conductivity than a typical coil formed of copper.